12,644 research outputs found
Replica analysis of overfitting in regression models for time-to-event data
Overfitting, which happens when the number of parameters in a model is too
large compared to the number of data points available for determining these
parameters, is a serious and growing problem in survival analysis. While modern
medicine presents us with data of unprecedented dimensionality, these data
cannot yet be used effectively for clinical outcome prediction. Standard error
measures in maximum likelihood regression, such as p-values and z-scores, are
blind to overfitting, and even for Cox's proportional hazards model (the main
tool of medical statisticians), one finds in literature only rules of thumb on
the number of samples required to avoid overfitting. In this paper we present a
mathematical theory of overfitting in regression models for time-to-event data,
which aims to increase our quantitative understanding of the problem and
provide practical tools with which to correct regression outcomes for the
impact of overfitting. It is based on the replica method, a statistical
mechanical technique for the analysis of heterogeneous many-variable systems
that has been used successfully for several decades in physics, biology, and
computer science, but not yet in medical statistics. We develop the theory
initially for arbitrary regression models for time-to-event data, and verify
its predictions in detail for the popular Cox model.Comment: 37 pages, 9 figure
Family farming in the agricultural census of 2006: the legal mark and the options for their identification.
(Agricultura familiar no censo agropecuário 2006: o marco legal e as opções para sua identificação). Visando delimitar a agricultura familiar no Censo Agropecuário 2006, o Ministério do Desenvolvimento Agrário (MDA) e o Instituto Brasileiro de Geografia e Estatística (IBGE) elaboraram metodologia para construção de uma variável identificando os estabelecimentos agropecuários recenseados e que se ajustam ao conceito previsto na Lei n.11.326, de 24 de julho de 2006. O texto apresenta os passos metodológicos utilizados e alguns resultados
Inspection and diagnosis tests for structural safety evaluation: A case study
Diagnosis and assessment of existing structures is a developing area due to the appearance of a high number of building defects, structural and non-structural deterioration and precocious loss of quality, and, consequently, lower expected durability. With the aim of verifying the viability of rehabilitation or the need to demolish an existing fifteen year old parking building, several inspections and diagnostic non-destructive and destructive testing, visual inspection, were carried out to evaluate the structural safety conditions
Mutual optical injection in coupled DBR laser pairs
We report an experimental study of nonlinear effects, characteristic of mutual optical coupling, in an ultra-short coupling regime observed in a distributed Bragg reflector laser pair fabricated on the same chip. Optical feedback is amplified via a double pass through a common onchip optical amplifier, which introduces further nonlinear phenomena. Optical coupling has been introduced via back reflection from a cleaveended fibre. The coupling may be varied in strength by varying the distance of the fibre from the output of the chip, without significantly affecting the coupling time. © 2008 Optical. Society of America
Sampling Twitter users for social science research: Evidence from a systematic review of the literature
All social media platforms can be used to conduct social science research, but Twitter is
the most popular as it provides its data via several Application Programming Interfaces,
which allows qualitative and quantitative research to be conducted with its members. As
Twitter is a huge universe, both in number of users and amount of data, sampling is generally
required when using it for research purposes. Researchers only recently began to question
whether tweet-level sampling—in which the tweet is the sampling unit—should be
replaced by user-level sampling—in which the user is the sampling unit. The major rationale
for this shift is that tweet-level sampling does not consider the fact that some core discussants
on Twitter are much more active tweeters than other less active users, thus causing
a sample biased towards the more active users. The knowledge on how to select representative
samples of users in the Twitterverse is still insufficient despite its relevance for reliable
and valid research outcomes. This paper contributes to this topic by presenting a systematic
quantitative literature review of sampling plans designed and executed in the context of
social science research in Twitter, including: (1) the definition of the target populations,
(2) the sampling frames used to support sample selection, (3) the sampling methods used
to obtain samples of Twitter users, (4) how data is collected from Twitter users, (5) the size
of the samples, and (6) how research validity is addressed. This review can be a methodological
guide for professionals and academics who want to conduct social science research
involving Twitter users and the Twitterverse.info:eu-repo/semantics/publishedVersio
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